A liquid organopolysiloxane (silicone rubber) composition which is flowabile at room temperature and which cures by heating or the like has been widely used as a potting material or coating material in various fields such as construction and industry.
An article produced by curing such liquid silicone rubber composition had a smooth and glossy surface, and when such composition is used for sealing, adhesion, or electric insulation of a liquid crystal display (LED), the surface of the cured composition reflects beams from inside and outside the LED. Such reflection resulted in the loss of image contrast which in turn resulted in the poor visibility, and also, in the poor reliability of the detection system receiving the light beam from the LED.
In order to cope with this problem, grinding of the surface of the cured article having a glossy surface was often carried out to thereby roughen the surface with a file. Also contemplated was increase in the luminance of the optical device. However, mechanical surface roughening of the cured article resulted in the increase in the number of steps, which invited increase in the production cost. Use of the optical device with higher luminance also resulted in the increased cost.
Another attempt has been incorporation of an inorganic filler such as pulverized silica or fumed silica having a specific gravity of 2.0 or higher in the liquid silicone rubber composition to thereby provide a cured article having a matted surface. However, incorporation of the inorganic filler at a high content was required in order to provide a sufficiently matted surface by this method, and such incorporation of the inorganic filler at a high content resulted in an increased viscosity of the composition, and hence, in a marked decrease in the flowability. Accordingly, use of such composition as a potting material or coating material was difficult.
Also proposed were use of liquid silicone rubber compositions containing a filler having a large particle size or a fiber-shaped filler for providing a cured article having a matted surface (see JP-A 6-157910 and JP-A 7-188560: Patent Documents 1 and 2).
In the case of these silicone rubber compositions, the filler had a specific gravity higher than that of the silicone rubber used for the base component, and these compositions were associated with the risk of filler separation during the storage. Prevention of such separation required increase in the filler content or increase in the viscosity of the base component, which sacrificed flowability of the composition.
Also proposed are liquid silicone rubber compositions having added thereto a small amount of hydrocarbon compound which is either liquid or solid at room temperature and which has poor compatibility with the silicone rubber (JP-A 2000-169711, JP-A 2000-80276, JP-A 2000-86896 both corresponding to U.S. Pat. No. 6,297,305, JP-A 2000-114600, JP-A 2000-129128, and JP-A 2000-129240 all corresponding to U.S. Pat. No. 6,268,432: Patent Documents 3 to 8). When these compositions are used, the hydrocarbon compound gradually migrates toward the surface of the cured article after the curing to thereby provide the cured article with a matted surface. Since the hydrocarbon is readily dispersible in the silicone rubber used as the base component but hardly separable from the silicon rubber due to the specific gravity of the hydrocarbon which is similar to that of the silicone rubber, the resulting composition will retain the flowability sufficient for its use in the potting application.
These silicone rubber composition, however, had a drawback that formation of the matted surface by the migration of the hydrocarbon to the surface of the cured article required time. Control of the degree of matting was also difficult since the degree of matting differed even if the hydrocarbon content in the composition were the same since migration proceeded at a different rate when surface area of the hydrocarbon per unit volume was different.
Another method known in the art is addition of a greatly excessive amount of a basic silane coupling agent such as γ-aminopropyltriethoxysilane in the composition. In this case, the basic silane coupling agent bleeds out from the surface during the curing of the composition, and production of a cured article having a matted surface is thereby enabled. This method, however, has the drawback that the large amount of silane coupling agent incorporated results in the inferior rubber property of the cured article.